It is customary in reaction injection molding (RIM) processes to inject metered amounts of two or more reactive chemicals at high pressure and elevated temperature into a cylindrical mixing chamber within the body of a mix head and immediately thereafter to discharge the mixed chemicals into a mold where reaction between the mixed chemicals continues to completion to form a molded polymeric article.
Separate passages are provided within the body of the mix head in communication with separate sources of the chemicals respectively. Typically, each passage has an adjustable nozzle at a discharge end in communication with the mixing chamber for discharging the chemical thereinto at a desired flow velocity during a mixing cycle. Where two such passages are provided for two reactive chemicals, the two nozzles usually discharge into the mixing chamber at diametrically opposed locations to effect high pressure and high velocity impingement mixing of the chemicals as they enter the mixing chamber. Upon completion of the mixing cycle, a reciprocatory plunger is operated in timed relationship to the flow of chemicals into the mixing chamber to expel the mixed chemicals from the chamber through an outlet into the mold.
As the plunger operates to expel the mixed chemicals from the mixing chamber, it simultaneously closes communication between the metering nozzles and the mixed chemicals and simultaneously opens communication between each metering nozzle and an associated recirculating duct, whereby the excess portion of each chemical is recirculated to its source without contacting or reacting with any other chemical. After expelling the mixed chemicals from the mixing chamber, the plunger is returned to its initial mixing position in which communication between the metering nozzles and their respective recirculating ducts is closed or blocked by the head of the plunger and communication is reopened between the mixing nozzles and the mixing chamber to enable initiation of the injection part of the cycle and repetition of the sequential part of the cycle as stated above.
Heretofore the nozzles have been made from steel and are subsequently heat treated to improve abrasion resistance. A number of problems have been encountered when using such steel nozzles, especially when a nozzle must be changed. For example, when the orifice thereof becomes worn or when a different nozzle is required to accommodate different chemicals or a change in operating conditions and removal of the nozzle is required, the nozzle may stick within its receptacle or support structure because a chemical has exuded, i.e., the chemical under pressure has worked its way, between the nozzle and its supporting structure during the high pressure RIM operation and then hardened. When difficulty is encountered in removing the nozzle, breakage of the nozzle may occur and necessitate removal of the mix head from the RIM molding apparatus for reworking and extraction of the nozzle.
The dimensional tolerance for the metering orifice of each nozzle must be closely maintained in order to control the rate of flow of chemicals accurately into the mixing chamber during the mixing cycle. Thus the problems of nozzle wear and replacement have become increasingly important with improving RIM technology utilizing higher mixing pressures and impingement velocities, particularly when reinforcing fillers such as glass fibers or particles are added to the reactive chemicals. The fillers not only require higher mixing pressure that can approximate 2000 psi or more to accommodate the resulting increased viscosity, which in itself increases abrasion, but such fillers also significantly increase abrasion or erosion of the metering orifice and reduce its useful life.
Also the increasing use of fillers in the fluid chemicals has materially increased the difficulty of obtaining optimum mixing of the reactant chemicals within the mixing chamber. Optimum mixing depends upon maintaining an optimum flow rate of each fluid reactant chemical through its metering orifice at an optimum pressure. Such optimum conditions differ for each reactive chemical and depend upon the effective size of the metering orifice, among other factors, including the reactive chemical flow rate to the metering orifice, which necessarily must equal the flow rate through that orifice.
Heretofore an adjustable metering stem in the reactive chemical supply passage to the metering orifice has been employed to compensate for orifice wear and to maintain a predetermined effective size for the metering orifice. In many situations the use of fillers and the consequent higher fluid pressures for the reactant chemical compositions render the use of an adjustable metering stem objectionable because the optimum flow conditions can be most readily maintained when a fixed or non-adjustable metering orifice is used. As a consequence of inevitable wear during high pressure abrasive operating conditions, the fixed orifice rapidly loses its ability for controlling flow velocity, such that replacement is required more frequently than for the less desirable adjustable orifices. Thus the difficulties encountered in nozzle changing have become increasingly significant.